1. Field of the Invention
The present invention relates to monitoring surface conditions and characteristics, such as contamination and corrosion and more particularly to the use of second-order nonlinear optics to perform surface sensitive measurements of these physical and chemical surface parameters with a high degree of specificity.
2. Description of the Related Art
Despite the plethora of surface diagnostics, there is no clear method which has shown superiority over all others. The most evident obstacle to implementation of these diagnostics is the requirement of operation in ultrahigh vacuum environments. The vacuum requirement precipitates from the fact that most surface sensitive analysis techniques involve scattering from the surface at a near-grazing incident angle. The scattering of electrons, x-rays, etc. in this geometry requires that nothing interfere with the input or output on its way to or from the surface. The use of optical interrogation would alleviate this requirement. However, linear optical phenomena are generally bulk phenomena and therefore suffer from a poor ability to discriminate surface characteristics.
In nonlinear optics, outputs are produced at sum, difference or harmonic frequencies of the input(s). Using second order nonlinear optical (NLO) surface spectroscopy to examine the physical properties and behavior of a surface or interface was originally proposed in the 1960""s, in xe2x80x9cLight Waves at the Boundary of Nonlinear Mediaxe2x80x9d by Bloembergen and P. S. Pershan, The Physical Review, 128, Page 193 (1962). Experimental work involving second harmonic generation was also performed. However, because lasers at the time were comparatively feeble, impractical, slow, etc., there was little subsequent work done on the development of second harmonic generation or, more generally, second order nonlinear optical (NLO) processes at surfaces until considerably later when lasers had evolved sufficiently that their use in an application was simpler.
Recently, researchers have reviewed NLO processing and concluded that lasers had developed enough that they could be used for studying the physical and chemical properties of surfaces and interfaces. For example, a theoretical study of the physics of the interface, and not its engineering aspects, has been performed. See Journal of Vacuum Science and Technology B, Volume 3, Number 5, September October 1985, Pages 1464-1466, Y. R. Shen, xe2x80x9cSurface Studies by Optical Second Harmonic Generation: an Overview.xe2x80x9d
U.S. Pat. No. 5,294,289, T. F. Heinz et al. discuss the use of second harmonic generation as a means to monitor the epitaxial growth of silicon semiconductor structures in a high vacuum chamber. Specifically, they examined the spectroscopic response at the interface between the electronically active silicon and the insulative layer of calcium fluoride. By monitoring the magnitude of the resonance, they could ascertain whether the insulator was present on the surface and whether it had electronically binded to the underlying semiconductor. The optical system that is employed involves the free space propagation of the laser from source to surface and to optical detector. There is no discussion of using optical fibers as the propagation path for inputs or outputs. At the time this patent was granted, optical fibers were not capable of handling high peak power optical pulse formats.
In U.S. Pat. No. 5,623,341, J. H. Hunt discusses the use of sum-frequency generation for the detection of contamination and corrosion on engine parts. In this incarnation, one of the inputs is a tunable IR beam that is tuned to a resonance of the contamination on the surface. The efficiency of the sum-frequency process is increased (so-called resonant enhancement) when the IR beam is resonant with a contaminant. If the contaminant is not present, there is no resonant enhancement. By comparing on and off resonant signals, the presence and level of contaminant can be deduced. The optical system that is employed involves the free space propagation of the laser from source to surface and to optical detector. There is no discussion of using optical fibers as the propagation path for inputs or outputs.
In U.S. Pat. No. 5,875,029, P. C. Jann et al. describe a versatile optical inspection instrument and method to inspect magnetic disk surfaces for surface defects. The device provides surface position information of the defects. However, the technique involves only linear optical processes. That is, the input and output light wavelengths are the same. There is also no discussion of optical fiber implementation.
In U.S. Pat. No. 5,883,714, Jann et al. describe a versatile optical inspection instrument and method to inspect magnetic disk surfaces for surface defects. The device is based on interferometric measurement and detects contaminants by measuring the Doppler shift in the light that results from scanning the light onto a contaminant or defect. By scanning, the device provides surface position information of the defects. However, the technique involves only linear optical processes and senses only phase changes. That is, the input and output light wavelengths are the same and there is no discussion of the use of optical fibers.
In U.S. Pat. No. 5,898,499, J. L. Pressesky discusses a system for detecting local surface discontinuities in magnetic storage discs. The device is an interferometric detector which scans the disc in a spiral motion. Local defects cause local changes in phase which are measured by interferometric techniques. This is a linear optical technique.
In U.S. Pat. No. 5,932,423, T. Sawatari et al. discuss a scatterometer for detecting surface defects in semiconductor wafers. This device is a linear interferometric device.
In U.S. Pat. No. 5,973,778, J. H. Hunt discusses the use of second harmonic generation for investigating molecular alignment within a thin polyimide film. The technique uses changes in the second harmonic polarization to determine surface molecular alignment. There is no discussion of the use of optical fibers as a propagation path for inputs or outputs. The nonlinear optical response of a semiconductor will be quite different than that of a liquid crystal film.
In U.S. Pat. No. 6,317,514 B1, S. Reinhorn et al. discuss a method and apparatus for inspecting a wafer surface to detect the presence of conductive material on the wafer. The device uses UV initiated electron emission to determine the location of conductive areas. Those areas which are metal will emit electrons. If the area, which is supposed to be conductive, is not, there will be no electron emission.
In U.S. Pat. No. 6,359,451 B1, G. N. Wallmark discusses a system for testing for opens and shorts between conductor traces on a circuit board. The technique uses electron scattering to perform its diagnostics and has no optics associated with it.
The present invention is a fiber-based nonlinear optical system for performing surface-sensitive spectroscopic characterizations on a surface to be interrogated. A first optical source provides a first input, the first optical source comprising a fixed wavelength laser. A first input optical fiber receives the first input and transmits the first input to a first position proximate a location on a surface to be interrogated. The first input fiber coupling optics receives the first input from an output end of the first input optical fiber and delivers the first input to the interrogated location on the surface. A second optical source provides a second input. The second optical source comprises a tunable wavelength laser. A second input optical fiber receives the second input and transmits the second input to a second position proximate the interrogated location. A second input fiber coupling optics receives the second input from an output end of the second input optical fiber and delivers the second input to the interrogated location on the surface. The first and second inputs are alignable so that their surface locations of optical illumination overlap on the interrogated location. Output fiber coupling optics receives a reflected output resulting from a three-wave mixing process occurring at the interrogated location. An output optical fiber receives an output of the output fiber coupling optics and transmits the output to a desired third position from the interrogated location. An output sensor system receives an output of the output optical fiber and collects, analyzes and interprets the output of the output optical fiber, wherein the first and second optical sources and the output sensor system may be disposed at desired distances and angles from the interrogated location.
When implementing an optical diagnostic, the laser and detector are often objects which are large or cumbersome or whose operation would benefit from remote location from the sample. If the laser or detector is located remotely, the optical inputs or outputs must be propagated to and from the surface via a series of mirrors, lenses and other optics. A line of sight must be maintained between source and surface and surface and detector, often causing inconvenience and increasing the complexity of implementation.
If, instead, one uses fiber optics, the propagation issues become simpler since the optical fiber can follow an arbitrary path to and from the surface. Optical fibers have recently become able to propagate high peak power pulses. Previously, attempts to do so would result in damage to the fiber.
Other objects, advantages, and novel features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.